1. Field of the Invention
The present invention generally relates to the storing of data on magnetic tape media and, more particularly, to systems that calibrate positioning errors during read and write operations on a tape drive.
2. Relevant Background
The market for mass storage devices is growing at what seems to be an ever increasing rate with the sales of high-performance computers penetrating numerous industries ranging from financial institutions to oil exploration companies. The processing power of these high-performance systems, and the data they generate, are increasing faster than the ability of storage devices to keep pace. The problem of data storage and rapid retrieval is particularly pronounced in computational-intensive applications which create huge amounts of data that need to be accessed in seconds rather than minutes, hours or even days.
Magnetic disks remain the preferred media for direct access to frequently used files because of their fast access times. However, because of their high cost per-unit of storage and limited capacity, magnetic disk recorders are prohibitively expensive and therefore impractical for large-scale data storage. With the advances in magnetic tape technology, tape based systems remain the preferred choice for mass data storage. In addition to cost, magnetic tape exceeds the storage density of almost any other medium, at least from a volumetric standpoint, because tape is a much thinner medium than, for example, magnetic disks, and tape can be tightly packed.
Magnetic tape is a magnetic recording medium made of a thin magnetizable coating on a long, narrow strip of plastic which is typically stored in the form of a spool on a cartridge or cassette. Typically, multiple, parallel data tracks may be written to and/or read data from the tape in one of a number of manners. In “linear” or “longitudinal” reading or recording, data is read or recorded by moving from a starting point on the first track of the tape and moving linearly down the tape along the first track or along a first path. Once the physical end of the tape (EOT) is reached, the tape is rewound to the beginning of the tape (BOT) at which point the read/record head assembly begins reading or recording linearly down the tape along the second track or along a second path. In a variation of the linear method known as “serpentine” reading and recording, the head assembly first spans the tape's entire length in one direction along one track or path and then returns in the opposite direction along an adjacent parallel track or path (and then continues sweeping back and forth along the tape). In another variation of the linear method known as “spiral in” reading and recording, the head assembly first spans the tape's entire length in a first direction along a first track or path adjacent one outer lateral edge of the tape, returns in the opposite direction along a second track or path adjacent the other lateral edge of the tape, spans the tape in the first direction again along a third track or path adjacent the first track, and so on in a spiral manner around the tape.
Often, magnetic tape is pre-formatted with a plurality of guards (i.e., strips of the tape on which user data cannot be written) running either perpendicular to the tape length (to separate the tape into a number of sections or segments) or along the tape length (to separate the tape into a number of servo portions or data bands on which independent read/write heads can operate). A head assembly may often have a plurality of read/write heads (e.g., each including a plurality of read or write elements) that are operable to read and/or write (e.g., simultaneously) in one or more tracks in respective data bands. One or more tracks written at the same time along the tape length may be referred to as a “wrap.” In this regard, a new wrap begins each time the head assembly begins reading or writing in the forward or reverse directions.